1. Field of Invention
The present invention relates to hydrostatic transmissions intended primarily for use in the lawn and garden industry on tractors, mowers, snow throwers and other types of lawn and garden machines.
2. Description of the Related Art
Hydrostatic transmissions typically are provided with a rotating pump driven by an internal combustion engine, which pumps oil to a fluid motor. Rotary motion of the motor is induced by the fluid being received in the motor, and is transferred to a drive axle for propelling the vehicle. The pump is of variable displacement, and pumps the fluid to the fixed displacement motor at variable rates, which varies the rotational speed of the motor, and through different conduits, which determines the direction of motor rotation. As pump displacement changes between positive and negative, or forward and reverse, positions about the pump's neutral position, to respectively forwardly and reversely drive the motor, these two fluid conduits alternatingly conduct high pressure driving fluid from the pump to the motor, and low pressure return fluid from the motor to the pump during transmission operation. Adjusting the rotating pump's displacement to zero provides a neutral mode in which no fluid is pumped to the motor, and rotation of the motor is thus not induced. Such transmissions are well-known in the art, and may have pumps and/or motors which are of radial or axial piston design, examples of such transmissions being disclosed in U.S. Pat. Nos. 5,373,697 (Jolliff et al.), 6,301,885 (Johnson et al.), and 6,422,109 (Jolliff et al.), the complete disclosures of which are all expressly incorporated herein by reference.
Common applications for hydrostatic transmissions, which includes hydrostatic transaxles, are vehicles such as tractors, riding mowers, large area walk-behind mowers, larger snow throwers, and other types of self-propelled lawn and garden machines. Although such machines are self-propelled, they must often be moved manually, usually when the pump is not being driven, in order to move the machine about within garages or other confined spaces. Additionally, the machine may at some time require being pushed or pulled in the event of failure, including running out of gas, while in operation.
Vehicles having hydrostatic transmissions generally resist being pushed or pulled without first mechanically disconnecting the motor from the axle which it drives, or breaking the operative fluid engagement between the pump and motor. Otherwise, back-driving the motor through rotation of the axle tends to pump the oil in the motor backwards toward the pump. If the nonoperating pump is adjusted to have some displacement, the fluid back-flowing through one of the conduits between the pump and motor attempts to rotate the pump at some speed in one direction or the other. However, because the pump is typically still coupled to the nonrotating driveshaft of the engine, through a belt and pulleys, for instance, it resists being rotated and thus the vehicle resists being moved. Even if the pump (adjusted to have some displacement) and engine were uncoupled, the oil's resistance to flow through the hydrostatic circuit, particularly at low temperatures, can make manually moving the vehicle difficult. Moreover, if the pump is adjusted to have zero displacement, the fluid pumped by the back-driven motor would not be able to rotate the pump and flow through it. The oil the back-driven motor attempts to pump thus has nowhere to go, and again the vehicle resists being moved.
Above-mentioned U.S. Pat. Nos. 6,301,885 and 6,422,109 disclose means for mechanically disconnecting an axial piston motor from a gear train through which the motor and axle are operatively engaged. However, to facilitate pushing or pulling of vehicles having hydrostatic transmissions without first mechanically disconnecting the motor from the axle, it is known to provide the transmission with some means for allowing the fluid pumped by the back-driven motor to be directed out of the motor or the conduits and to an oil sump, rather than to allow this fluid to urge rotation of the pump. These means often include relief or dump valves between the fluid conduits and the oil sump. When opened, these valves allow fluid being pumped through either conduit (by the back-driven motor or perhaps by the pump) to freely flow into the sump, substantially emptying the hydrostatic circuit of fluid, and thereby taking the pump and motor out of operative fluid engagement with each other. Some embodiments of these dump valves are ball check valves which are normally biased by springs and/or pressurized fluid in the conduits into a closed, sealed state, and which are forced off their seats to place the conduits and the sump in fluid communication, thereby allowing fluid pumped by the back-driven motor to be expelled from the conduit(s) to the sump, and taking the pump and motor out of operative engagement. Such check valves also allow makeup fluid to be automatically drawn from the sump into the lower pressure conduit of the hydrostatic circuit as needed during transmission operation.
Above-mentioned U.S. Pat. No. 5,373,697 discloses (FIGS. 1 and 8) a bypass mechanism or hydraulic disconnect mechanism comprising a rigid metal plate which lies in a plane longitudinally-bisecting the pintle about which the radial piston pump and motor are disposed, and in which the two conduits are provided. Ball check valves are forced off of their seats by moving the rigid metal plate along the longitudinal axis of the pintle, fingers formed in the rigid plate being brought into engagement with the balls. Movement of the rigid metal plate is controlled manually, by rotating a rod extending normally through an oblong slot provided in the plate, the rod having an eccentric portion which slidably engages the edges of the slot. This bypass or hydraulic disconnect mechanism provides the advantage of being simple and easily-actuated manually, but may not be conveniently packaged in an axial piston-type hydrostatic transmission such as disclosed in above-mentioned U.S. Pat. Nos. 6,301,885 and 6,422,109. Such axial piston-type transmissions typically have a housing which partially defines a sump, and a center section or block to which the pump and motor are both rotatably attached. The center section has the fluid conduits formed therein and, in its bottommost surface, ports vertically extending to the conduits and containing check valves for allowing makeup fluid to enter the low pressure conduit from the sump during transmission operation. The space between the bottom surface of the center section block and the interior surface of the bottom wall of the housing is normally minimized to reduce transmission height, and affords insufficient room for packaging a vertically-oriented and vertically-moving rigid metal plate like that disclosed in U.S. Pat. No. 5,373,697.
Prior means for providing a hydraulic bypass or disconnect mechanism in an axial piston-type hydrostatic transmission are disclosed in U.S. Pat. No. 6,332,317 (Hauser et al.) which, as described above, provides ball check valves being placed in vertically-extending ports in the bottom surface of the center block, through which the conduits and sump may be placed in fluid communication. The bypass mechanisms of U.S. Pat. No. 6,332,317 provide non-manual means for pushing the balls upwardly off their seats with a horizontally-sliding plate moved by an electrical solenoid, or with an electromagnetically—actuated, vertically-moving horizontal plate or a ball-pushing member, or by electromagnetically lifting the steel balls themselves off of their seats. Although some of these hydraulic disconnect mechanism embodiments may be packaged in the small space normally available in the sump beneath the center section and above the interior surface of the bottom wall of the transmission housing, they all have the disadvantage of requiring electrical power, which may not always be available, and of being somewhat complex. U.S. Pat. No. 6,332,317 neither teaches nor suggests any means for manually-actuating the hydraulic bypass mechanisms it discloses.
A simple, manually-actuated hydraulic disconnect or bypass mechanism which does not rely on electrical power, and/or which may be suitably packaged in the sump, in the space between the center section and the housing wall in an axial-piston type hydrostatic transmission, is desirable.